Fields of Disclosure
The disclosure relates generally to the field of fluid separation. More specifically, the disclosure relates to a method and system of controlling a temperature within a melt tray assembly of a distillation tower.
Description of Related Art
This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The production of natural gas hydrocarbons, such as methane and ethane, from a reservoir often carries with it the incidental production of non-hydrocarbon gases. Such gases include contaminants, such as at least one of carbon dioxide (“CO2”), hydrogen sulfide (“H2S”), carbonyl sulfide, carbon disulfide, and various mercaptans. When a stream being produced from a reservoir includes these contaminants mixed with hydrocarbons, the stream is oftentimes referred to as “sour gas.”
Many natural gas reservoirs have relatively low percentages of hydrocarbons and relatively high percentages of contaminants. Contaminants may act as a diluent and lower the heat content of the produced hydrocarbon stream. Some contaminants, like sulfur-bearing compounds, can also be noxious and, in the presence of liquid water, some contaminants can be corrosive to carbon steel. Therefore, it is desirable to remove contaminants from a stream containing hydrocarbons to produce sweet and concentrated hydrocarbons.
Specifications for pipeline quality natural gas typically call for a maximum of 2-4% CO2 and ¼ grain H2S per 100 standard cubic feet (scf) (4 parts per million volume (ppmv)) or 5 milligrams per Normal meter cubed (mg/Nm3) H2S. Specifications for lower temperature processes such as natural gas liquefaction plants or nitrogen rejection units typically require less than 50 parts per million (ppm) CO2.
The separation of contaminants from hydrocarbons is difficult and consequently significant work has been applied to the development of hydrocarbon/contaminant separation methods. These methods can be placed into three general classes: absorption by solvents (e.g., physical, chemical, and hybrids), adsorption by solids, and distillation.
Separation by distillation of some gas mixtures can be relatively simple and, as such, is often widely used in the natural gas industry. However, distillation of mixtures of natural gas hydrocarbons, primarily methane, and one of the most common contaminants in natural gas, carbon dioxide, can present significant difficulties. Conventional distillation principles and conventional distillation equipment are predicated on the presence of only vapor and liquid phases throughout the distillation tower. The separation of CO2 from methane by distillation involves temperature and pressure conditions that result in solidification of CO2 if pipeline quality or better hydrocarbon product is desired. The required temperatures are cold temperatures typically referred to as cryogenic temperatures.
Certain cryogenic distillations can overcome the above mentioned difficulties. These cryogenic distillations provide the appropriate mechanism to handle the formation and subsequent melting of solids during the separation of solid-forming contaminants from hydrocarbons. The formation of solid contaminants in equilibrium with vapor-liquid mixtures of hydrocarbons and contaminants at particular conditions of temperature and pressure takes place in a controlled freeze zone section of a distillation tower.
The controlled freeze zone section comprises a melt tray assembly. The melt tray assembly collects and warms solids that form in the controlled freeze zone section. Liquid in the melt tray assembly helps conduct heat to warm the solids and create a liquid slurry. This liquid and/or the liquid slurry may alternately be referred to herein as a liquid bath. The melt tray assembly provides adequate heat transfer to melt the solids and facilitate liquid slurry draw-off to a stripper section of the distillation tower.
Maintaining the liquid bath in the melt tray assembly at a generally uniform and/or steady-state conditions is important for overall process stability within the distillation tower. Too high of a temperature can result in decreased separation performance of the contaminants from the stream containing the hydrocarbons in the controlled freeze zone section, which in turn can result in higher contaminant content in the stream flowing through a rectifier section of the distillation tower and/or can lead to solid formation in the rectifier section. Solid formation in the rectifier section can cause a disruption within the distillation process and prevent adequate removal of the contaminants from the stream. Too low of a temperature can result in solid formation in the melt tray assembly, which can stop flow of the liquid slurry into the stripper or lower distillation section, thereby disrupting operation within the distillation process. An unsteady melt tray assembly temperature can negatively affect the rate of removal of contaminants from the stream in the controlled freeze zone section. The purity of the hydrocarbons sold may be detrimentally affected. Operation costs of the distillation process may increase.
The melt tray heat exchange device is used to facilitate the warming of the solids, formed by a spray assembly in the controlled freeze zone section, in the melt tray assembly. Conventional controlled freeze zone sections comprise a melt tray assembly with a melt tray heat exchange device having coils of tubing designed to pass a warm heat medium fluid into the liquid bath across a substantial portion of the liquid bath. Some designs include vapor risers with a bubble cap at an upper end an example of which is described in U.S. Pat. No. 5,265,428, incorporated herein by reference.
Disadvantages can result when using the above described approaches. For example, the above described approaches can develop pockets within the liquid bath with relatively higher or lower temperatures or otherwise can make it difficult to obtain generally uniform heat transfer within the melt tray assembly. In designs releasing vapor into the bottom of the liquid bath, misdistribution of the warm vapor bubbles entering the bottom of the liquid bath across its available volume reduces the liquid bath's thermal effectiveness to maintain a constant temperature that will effectively and evenly melt collected solid material. Additionally, bubble cap designs contain a surface that is ineffective for heat transfer, namely, an inner wall of the vapor riser.
A need exists for improved technology that can better facilitate heat transfer within the melt tray assembly and/or liquid bath, that effectively mixes warm vapor across the available liquid bath volume, that increases the effectiveness of and/or more efficiently utilizes the available surface area between the vapor riser and the liquid bath, that reduces the compartmentalization of heat supplied to the liquid bath, and/or that minimizes the lateral temperature differential within the melt tray assembly and/or liquid bath.